Method for detecting the space orientation and position of an object

ABSTRACT

The invention relates to a method for the optical detection of the position and orientation of an object by means of an optical device comprising at least one parallelogram fastened to said object, the optical device comprising optical means and electronic analysis means making it possible to determine the coordinates of the four vertices of the parallelogram A′B′C′D′, in an orthonormal frame with center  0 , denoted R 0  (O,{right arrow over (i)},{right arrow over (j)},{right arrow over (k)}). The principle of the device consists in determining the vertices of the parallelogram A′B′C′D′, on the basis of the knowledge of the characteristics of the parallelogram and of four known points of a quadrilateral ABCD. This quadrilateral represents the drawing arising from the projection of the parallelogram A′B′C′D′ in a known image plane. The characteristics A′B′C′D′ of the parallelogram can be for example its height, its width and the coordinate of one of its points in the frame R 0 .

The present invention relates to a method for the optical detection ofthe position and orientation of an object in space. It applies moreparticularly in the aeronautical field. In this case, the object is apilot's helmet comprising a helmet viewing system.

The determination of the position and orientation of an object in spaceis a problem relating to numerous technical fields. The varioussolutions generally afforded must have the principal characteristics ofresolving any ambiguity in position or orientation, of responding tomore or less severe dynamics of the systems and of satisfying highaccuracy.

These systems are used in aeronautics, for detecting head posture,notably for the helmets of fighter aircraft, of military, civilian orpara-civilian helicopters. They are also used for detecting simulationhelmets, this detection can then be combined with an oculometry device,also called an eyetracker, for detecting position of the gaze. Numerousapplications of these systems also exist in the field of virtual realityand games.

Currently, optical systems for detecting posture rely on two mainprinciples. Firstly, it is possible to identify on an image, produced bya matrix sensor for example, the position of luminous pointlikeemitters. Electroluminescent diodes, also called LEDs, can be used asemitters. Additionally, another solution consists in observing anunambiguous pattern printed on the object whose position and orientationare to be determined. For this purpose, one or more cameras are used toobserve this pattern and analyze it on the basis of the imagescollected.

In the case of the use of luminous sources of the LED type, the latterare disposed in groups. These groups of LEDs are also called clusters.In the case of aeronautical applications, these clusters, disposed onthe helmet, are generally not contained in a plane, and in numerouscases take the form of a tetrahedron on the helmet.

FIG. 1 represents a helmet 1 used in aeronautics for systems fordetecting the position and orientation of objects in space. The diodes10 placed on the helmet form a tetrahedron-shaped cluster. Thetetrahedron is indicated by dashed lines in FIG. 1. This type of systemrequires sensors, generally cameras placed in the cockpit. This entailsa multi-emitter/multi-receiver device whose emitters are the diodes andthe receivers the cameras.

The analysis of the information arising from the sensors is complex,having regard to the spatial geometry which requires large computationalcapabilities. Additionally, the slaving of a system of this type mayexhibit aspects that are limiting in terms of fastness of thecomputation time and may therefore affect the accuracy of the systems.To attain the required accuracy, the sensor, of camera type, must have ahigh resolution and the processing of the sensor information is subjectto a prediction of the position of the LEDs and an analysis of zones ofinterest.

Variants of these systems exist, notably devices for detecting theshadow of a grid illuminated by a helmet-mounted source. These systemsexhibit a limitation on the accurate determination of the orientation ofthe object to be identified.

The process of detecting the position and orientation of an object,through the observation of a pattern on said object by cameras, is lessaccurate. This process requires large computational capabilities andposes problems of use in disturbed environments. One way of improvingthe performance of such a system is to multiply the sensors and to placethem in an optimal manner. This solution nevertheless remains difficultto implement.

In a general manner, the current solutions for detecting the positionand orientation of an object in space, in the aeronautical field,exhibit limitations related to the compromise between the implementationof computationally extremely unwieldy solutions and the accuracyrequirements demanded. Additionally, the constraints of the aeronauticalenvironment necessitate a redundancy of the optical means or of thesensors and do not allow the implementation of simple technicalsolutions.

The method according to the invention makes it possible, notably, toalleviate the aforesaid drawbacks. Specifically, the device comprisessensors or emitters grouped into clusters having a parallelogram shape.The method for determining the position of the sensors is, therefore,simple to implement and requires very few computations, the method beingdeterministic. This method is very advantageous in the case of slavedsystems where the times between two measurements are reduced and theaccuracy of detection increased.

Advantageously, the method for the optical detection of the position andorientation of an object is carried out by means of an optical devicecomprising at least one first parallelogram (A′B′C′D′) fastened to saidobject and comprising optical means and electronic analysis means makingit possible to determine the coordinates of the four vertices of thefirst parallelogram (A′B′C′D′), in an orthonormal frame (R₀(O,{rightarrow over (i)},{right arrow over (j)},{right arrow over (k)})),comprising a center (O), said frame comprising a plane (O,{right arrowover (j)},{right arrow over (k)}) parallel to the image plane (Pi). Theimage plane is without ambiguity the image plane of the optical deviceconsidered.

The method comprises several steps:

-   -   a first step of defining a second reference parallelogram        (A₀B₀C₀D₀) whose center (O) is the center of the frame        (R₀(O,{right arrow over (i)},{right arrow over (j)},{right arrow        over (k)})), possessing the same characteristics as the first        parallelogram (A′B′C′D′), situated in the plane (O,ĵ,{circumflex        over (k)}) parallel to the image plane (Pi);    -   a second step of defining the transformation under which the        first parallelogram (A′B′C′D′) is the image of the second        parallelogram (A₀B₀C₀D₀), said transformation decomposing into a        translation {right arrow over (u)} and a vector rotation r.    -   a third step of determining, through the optical means, a        quadrilateral (ABCD), obtained by projecting the first        parallelogram (A′B′C′D′) into the image plane (Pi), with nonzero        abscissa Xi, in the frame (R0) with center 0, along a direction        ({right arrow over (i)}) perpendicular to the image plane (Pi),    -   a fourth step of determining:        -   a first point (E) belonging to the image plane (Pi), when it            exists, such that the first point (E) is the intersection of            the straight lines formed by two opposite sides of the            quadrilateral (AB, CD);        -   a second point (F) belonging to the image plane (Pi), when            it exists, such that the second point (F) is the            intersection of the straight lines formed by the other two            sides of the quadrilateral (AC, BD),        -   a first vector ({right arrow over (OE)}), connecting the            center of the frame (O) and the first point (E);        -   a second vector ({right arrow over (OF)}), connecting the            center of the frame (O) and the second point (F);    -   a fifth step of determining the respective images of the unit        vectors ({right arrow over (i)},{right arrow over (j)},{right        arrow over (k)}), defining the frame (R₀), by the rotation r, as        a function of the first and second vectors ({right arrow over        (OE)},{right arrow over (OF)}) and of the coordinates of the        four vertices (A0,B0,C0,D0) of the second parallelogram        (A0B0C0D0);    -   a sixth step of determining the translation {right arrow over        (u)} as a function of the first and second vectors ({right arrow        over (OE)},{right arrow over (OF)}) and of the coordinates of        the four vertices (A₀,B₀,C₀,D₀) of the second parallelogram        (A₀B₀C₀D₀). The knowledge of the translation {right arrow over        (u)} and of the rotation r suffices to pinpoint the position of        the object, as well as its attitude in space.    -   finally, a seventh step may be carried out to determine the        coordinates of the vertices of the first parallelogram        (A′,B′,C′,D′) in R₀, on the basis of the known coordinates of        the vertices of the second parallelogram (A0,B0,C0,D0) and of        the transformation composed of a translation u and of a rotation        r.

Advantageously, the detection method can comprise particular forms ofparallelograms such as diamonds, rectangles or squares.

Advantageously, the optical detection method can comprise optical meanscomprising a holographic video-projector emitting, in an image, sharpluminous patterns at every point of the zone of sweep of said object andat least two identical and mutually parallel lineal matrix sensors,disposed on the object, the four ends of these two sensors forming aparallelogram.

Advantageously, the optical detection method can comprise optical meanscomprising a camera and at least four emitting diodes disposed on theobject, each of which represents the ends of a parallelogram.

Other characteristics and advantages of the invention will becomeapparent with the aid of the description which follows given with regardto the appended drawings which represent:

FIG. 1 represents a pilot's helmet according to the state of the art;

FIG. 2 represents the characteristics of a reference parallelogram;

FIG. 3 represents a 3D view of the drawing of a parallelogram arisingfrom its projection in an image plane;

FIG. 4 represents the vanishing points of the drawing of FIG. 3, whenthey exist;

FIG. 5 represents the vectors, known in Ro, of the vanishing points ofthe drawing of FIG. 3;

FIG. 6 represents an exemplary optical device according to theinvention.

The optical detection method according to the invention consists indetermining the vertices of a parallelogram A′B′C′D′, situated in aframe R₀ of space, denoted R₀(O,{right arrow over (i)},{right arrow over(j)},{right arrow over (k)}), on the basis of the knowledge of thecharacteristics of the parallelogram and of four known points of aquadrilateral ABCD. This quadrilateral represents a drawing arising fromthe projection of the parallelogram A′B′C′D′ in an image plane.

The characteristics A′B′C′D′ of the parallelogram may be for example itsheight, its width and the coordinate of one of its points in the frameR₀. Of course, any other mode of representation could be appropriate.

This detection of the parallelogram is done by means of an opticaldevice making it possible, when the parallelogram is fixed to an object,to pinpoint the position and the orientation of the object in R₀.

FIG. 2 shows an example of a parallelogram 20 with vertices A₀, B₀, C₀and D₀ and whose characteristics are the same as those of theparallelogram A′B′C′D′ whose position and orientation in R₀ are to bedetermined. The parallelogram 20 possesses four sides denoted A₀B₀,C₀D₀, A₀C₀ and B₀D₀ that are pairwise mutually parallel. The height 21of the parallelogram is denoted H, its width 22 is denoted L and thecoordinate 23 of A₀ in the frame R₀ along {right arrow over (j)} isdenoted T.

The four points are defined in R₀ by the following equations:

${\overset{\rightarrow}{{OA}_{0}} = {{T\overset{\rightarrow}{j}} + {\frac{H}{2}\overset{\rightarrow}{k}}}},{\overset{\rightarrow}{{OB}_{0}}\mspace{45mu} = {{\left( {T - L} \right)\overset{\rightarrow}{j}} + {\frac{H}{2}\overset{\rightarrow}{k}}}},{\overset{\rightarrow}{{OC}_{0}}\mspace{45mu} = {{{- \overset{\rightarrow}{{OB}_{0}}}\mspace{14mu} {and}\mspace{14mu} \overset{\rightarrow}{{OD}_{0}}}\mspace{45mu} = {- {\overset{\rightarrow}{{OA}_{0}}.}}}}$

As indicated in FIG. 3, this reference parallelogram is placed in theframe R₀, in such a manner that its center is O. The plane (O,{rightarrow over (j)},{right arrow over (k)}) denoted P₀ is parallel to theplane P_(i) denoted (X_(i),{right arrow over (j)},{right arrow over(k)}), the latter being the image plane. The plane P_(i) contains thedrawing ABCD of the quadrilateral where X_(i) is the abscissa of theplane along the axis {right arrow over (i)}.

It is equivalent to know the coordinates of the four vertices of theparallelogram A′B′C′D′ in R₀ as to know the analytical transformationwhich makes it possible to deduce A′B′C′D′ from the parallelogram 20.

Given that the two parallelograms have the same characteristics, thereexists a direct vector rotation r in relation to an axis passing throughO and a translation {right arrow over (u)}, r and {right arrow over (u)}being unique, such that,

{right arrow over (OA′)}={right arrow over (u)}+r({right arrow over (OA₀)})

{right arrow over (OB′)}={right arrow over (u)}+r({right arrow over (OB₀)})

{right arrow over (OC′)}={right arrow over (u)}+r({right arrow over (OC₀)})

{right arrow over (OC′)}={right arrow over (u)}+r({right arrow over (OD₀)})

FIG. 3 represents the parallelogram 30 denoted A′B′C′D′, in the frameR₀. Its drawing 31, arising from the projection of A′B′C′D′ in the planeP_(i) is represented by the quadrilateral ABCD.

The coordinates of the quadrilateral ABCD in R₀ being known through theoptical detection method, the algorithm makes it possible on the basisof the drawing 31 and of the reference parallelogram 20, to ascertainthe transformations r and {right arrow over (u)}. The position andattitude of the object can be deduced from r and {right arrow over (u)}directly, without specifically knowing the positions of the vertices ofthe parallelogram A′B′C′D′.

FIG. 4 represents in the plane P_(i), the quadrilateral ABCD. When theyexist, this corresponding to the most frequent case, the coordinates ofthe points of intersection of the straight lines (AB) and (CD) and ofthe straight lines (AD) and (BC) are determined by knowing thecoordinates of the points A, B, C, D in R₀. The point of intersection ofthe straight lines (AB) and (CD) is then denoted E and the point ofintersection of the straight lines (AD) and (BC) is denoted F. In thiscase, the vector {right arrow over (OE)} is denoted {right arrow over(e)} and the vector {right arrow over (OF)} is denoted {right arrow over(f)}.

It is known that the vector {right arrow over (e)} is positivelyproportional to {right arrow over (A′B′)} and that the vector {rightarrow over (f)} is positively proportional to {right arrow over (A′C)}in R₀.

FIG. 5 represents when they exist, the vectors {right arrow over (OE)}and {right arrow over (OF)} in the frame R₀ and illustrates theaforesaid property.

The cases, where E does not exist or F does not exist or E and F do notexist, correspond, respectively, to the following relations, which ensuefrom the geometry of the quadrilateral ABCD:

-   -   the sides AB and CD are parallel. ABCD is a trapezium in        relation to AB, that is to say the side A′B′ is parallel to the        image plane and the side A′C′ is not. We determine {right arrow        over (e)}={right arrow over (AB)} and {right arrow over        (f)}={right arrow over (OF)}.    -   the sides BC and AD are parallel, ABCD is a trapezium in        relation to BC, that is to say the side A′C′ is parallel to the        image plane and the side A′B′ is not; We determine {right arrow        over (f)}={right arrow over (AC)} and {right arrow over        (e)}={right arrow over (OE)}.    -   ABCD is a parallelogram, that is to say the parallelogram        A′B′C′D′ is parallel to the image plane. We have the following        two relations: {right arrow over (e)}={right arrow over (AB)}        and {right arrow over (f)}={right arrow over (AC)}.

The following computations are carried out in the case where E and Fexist, the simplifications being made naturally for the particular caseswhere a determined solution exists for each case.

From {right arrow over (A′B′)}=r({right arrow over (A₀B₀)}), we obtain,with the previous notation: {right arrow over (A′B′)}=−L·r({right arrowover (j)}), we deduce that given that

${{r\left( \overset{\rightarrow}{j} \right)} = {- \frac{\overset{\rightarrow}{e}}{\overset{\rightarrow}{e}}}},$

given that ∥{right arrow over (j)}∥=1

r({right arrow over (j)})∥=1.

Likewise, from {right arrow over (A′C′)}=r({right arrow over (A₀C₀)}),we obtain, with the previous notation: {right arrow over(A′C′)}=(L−2T)·r({right arrow over (j)})−Hr({right arrow over (k)}),

We deduce that:

${{r\left( \overset{\rightarrow}{k} \right)} = {{\frac{q}{H}{r\left( \overset{\rightarrow}{j} \right)}} - {\frac{\sqrt{q^{2} + H^{2}}}{H}\frac{\overset{\rightarrow}{f}}{\overset{\rightarrow}{f}}}}},$

where q=L−2T.

r being a direct rotation, we obtain: r({right arrow over (i)})=r({rightarrow over (j)})

r({right arrow over (k)}).

The three respective images of {right arrow over (i)},{right arrow over(j)},{right arrow over (k)} under the rotation r are determined as afunction of the known characteristics of the parallelogram and of thetwo vectors {right arrow over (e)} and {right arrow over (f)}.

From {right arrow over (OA′)}={right arrow over (u)}+r({right arrow over(OA₀)}), we derive

$\overset{\rightarrow}{u} = {\overset{\rightarrow}{{OA}^{\prime}} - {T \cdot {r\left( \overset{\rightarrow}{j} \right)}} - {\frac{H}{2} \cdot {{r\left( \overset{\rightarrow}{k} \right)}.}}}$

If μ_(E) denotes the known real such that {right arrow over(AE)}=μ_(E){right arrow over (AB)} and k denotes the real such that{right arrow over (OA′)}=k·{right arrow over (OA)}, it then followsthat:

$k = {\frac{A^{\prime}B^{\prime}}{OE} \cdot {{{\mu_{E} - 1}}.}}$

In the same manner we have μ_(F) the real defined by the relation {rightarrow over (AF)}=μ_(F){right arrow over (AC)}.

The analytical result for the sought-after translation is obtained:

${\overset{\rightarrow}{u} = {{k \cdot \overset{\rightarrow}{OA}} - {T \cdot {r\left( \overset{\rightarrow}{j} \right)}} - {\frac{H}{2} \cdot {r\left( \overset{\rightarrow}{k} \right)}}}},$

with k known.

The parallelogram A′B′C′D′ is deduced by determining the transformationcomposed of a known vector rotation and of a known translation, of thereference parallelogram A₀B₀C₀D₀.

In the case where A′B′C′D′ is a diamond we have additional relation:|1−μ_(E)|·OF=|1−μ_(F)·OE.

In the case where A′B′C′D′ is a rectangle we have additional relation:({right arrow over (OE)}·{right arrow over (OF)})=0.

In the case where A′B′C′D′ is a square, the analytical expressions forthe transformations of {right arrow over (i)},{right arrow over(j)},{right arrow over (k)} under the rotation r are simplified. Weobtain: L=H=2×T and the rotation of the vector {right arrow over (k)} isdetermined in a simple manner:

${r\left( \overset{\rightarrow}{k} \right)} = {- {\frac{\overset{\rightarrow}{f}}{\overset{\rightarrow}{f}}.}}$

The two additional relations, corresponding to the case of the diamondand of the rectangle, are both valid for the case of the square.

FIG. 6 is an exemplary device, according to the invention. An object 65comprising electro-optical receivers, of lineal matrix sensor type and ameans for projecting images, said images comprising luminous patterns61.

The sensors are grouped together in such a manner that pairwise theyform parallelograms 30. The sensors are pairwise mutually parallel andof equal size.

Additionally, an exemplary means for projecting images, according to theinvention, is to use an optical means emitting, at every point of thezone of sweep 66 of the object 65, a sharp image. The sensors placed onthe helmet receive unambiguous signals originating from this image.

For this purpose, an exemplary embodiment of the invention uses aholographic video-projector 60 as projection means. Holographicvideo-projectors such as these are produced and marketed by the companyLight Blue Optics and are known under the brand PVPro. This holographicvideo-projector possesses the advantageous property of emitting a sharpimage at every point of the zone of sweep 66.

This holographic video-projector, called VPH hereinafter, comprises acoherent light source, which is generally a laser diode, a displaymaking it possible to produce a phase image, optical means arranged soas to create on the basis of the wave emitted by the light source, afirst reference wave and a second wave modulated by the display andmeans allowing these two waves to be made to interfere. The final imageobtained is a Fraunhofer hologram of the phase image generated on thedisplay. It is possible to generate any type of image by this means. Thedisplay may be a liquid crystal display, of LCOS type for example.

Under these conditions, the center O of the frame R₀ is defined by apoint of the VPH, and the plane (O,{right arrow over (j)},{right arrowover (k)}) is the image plane parallel to the image plane 32 of theprojected image comprising the origin.

So as to pinpoint the object in space, the VPH emits images comprisingluminous patterns 61 on the sensors situated on the helmet. The analysisof the information arising from the sensors is carried out by a digitalcomputer 64, placed downstream of the sensors, in the processing chainfor treating the signals received.

The analysis of the signals received by each cell makes it possible toreconstitute the drawing, obtained by projection of the parallelogrampositioned on the object in the image plane. The drawing is determined,in an almost natural manner, by photographing the patterns deformed inthe local plane of the parallelogram. Knowing the original patterns andtheir deformations identified by the sensors, the a priori knowledge ofthe characteristics of the parallelogram give us its drawing inversely.The latter represents a quadrilateral in the image plane.

On the basis of this drawing, and of the knowledge of thecharacteristics of the parallelogram, a priori known, the method makesit possible to retrieve in a simple manner the position and orientationof the cluster in the frame R₀.

A second variant embodiment is to consider an optical device comprisingat least one camera and a pilot's helmet comprising emitting diodesgrouped into clusters At least one cluster forms a parallelogramA′B′C′D′, whose vertices are diodes.

Under these conditions, the zone of sweep is all or part of the cockpit.

The center of the frame R₀ is the camera, the plane (O,{right arrow over(j)},{right arrow over (k)}) is the image plane of the camera. Thecamera then obtains, in its image plane, the representation of thequadrilateral ABCD arising from the projection of the parallelogramA′B′C′D′ in the image plane.

The analysis means can therefore retrieve the position and orientationof the cluster on the basis of the knowledge of the representation ofthe quadrilateral in a known image plane and of the a priori knowncharacteristics of the parallelogram.

1. A method for the optical detection of the position and orientation ofan object by means of an optical device comprising at least one firstparallelogram (A′B′C′D′) fastened to the object, whose characteristicsare known, and having optical means and electronic analysis means makingit possible to determine the coordinates of the four vertices of thefirst parallelogram (A′B′C′D′), in an orthonormal frame (R₀(O,{rightarrow over (i)},{right arrow over (j)},{right arrow over (k)})), havinga center (O), said frame comprising a plane (O,{right arrow over(j)},{right arrow over (k)}) parallel to the image plane (Pi), saidmethod comprising: a first step of defining a second referenceparallelogram (A₀B₀C₀D₀) whose center (O) is the center of the frame(R₀(O,{right arrow over (i)},{right arrow over (j)},{right arrow over(k)})), possessing the same characteristics as the first parallelogram(A′B′C′D′), situated in the plane (O,{right arrow over (j)},{right arrowover (k)}) parallel to the image plane (Pi); a second step of definingthe transformation under which the first parallelogram (A′B′C′D′) is theimage of the second parallelogram (A₀B₀C₀D₀), said transformationdecomposing into a translation {right arrow over (u)} and a vectorrotation r. a third step of determining, through the optical means, aquadrilateral (ABCD), obtained by projecting the first parallelogram(A′B′C′D′) into the image plane (Pi), with nonzero abscissa Xi, in theframe (R0) with center 0, along a direction ({right arrow over (i)})perpendicular to the image plane (Pi), a fourth step of determining: afirst point (E) belonging to the image plane (Pi), when it exists, suchthat the first point (E) is the intersection of the straight linesformed by two opposite sides of the quadrilateral (AB, CD); a secondpoint (F) belonging to the image plane (Pi), when it exists, such thatthe second point (F) is the intersection of the straight lines formed bythe other two sides of the quadrilateral (AC,BD), a first vector ({rightarrow over (OE)}), connecting the center of the frame (O) and the firstpoint (E); a second vector ({right arrow over (OF)}), connecting thecenter of the frame (O) and the second point (F); a fifth step ofdetermining the respective images of the unit vectors ({right arrow over(i)},{right arrow over (j)},{right arrow over (k)}), defining the frame(R₀), by the rotation (r) of the transformation, as a function of thefirst and second vectors ({right arrow over (OE)},{right arrow over(OF)}) and of the known characteristics of the second parallelogram(A₀B₀C₀D₀); a sixth step of determining the translation ({right arrowover (u)}) of the transformation as a function of the first and secondvectors ({right arrow over (OE)},{right arrow over (OF)}), of the vectorconnecting the center of the frame (O) to a vertex of the quadrilateral(ABCD) and of the known characteristics of the second parallelogram(A₀B₀C₀D₀).
 2. The detection method as claimed in claim 1, comprising aseventh step of determining the coordinates of the vertices (A′, B′, C′,D′) of the first parallelogram in the frame (R₀), as a function of theknown coordinates of the vertices of the second parallelogram (A₀B₀C₀D₀)and of the transformation composed of a translation ({right arrow over(u)}) and of a rotation (r).
 3. The detection method as claimed in claim1, wherein the parallelogram is a diamond.
 4. The detection method asclaimed in claim 1, wherein the parallelogram is a rectangle.
 5. Thedetection method as claimed in claim 1, wherein the parallelogram is asquare.
 6. The method of optical detection as claimed in claim 1,wherein the device comprises optical means comprising a holographicvideo-projector emitting in an image plane sharp luminous patterns atevery point of the zone of sweep corresponding to the space in which theobject may move and at least two identical and mutually parallel linealmatrix sensors, disposed on the object, the four ends of these twosensors forming a parallelogram.
 7. The method of optical detection asclaimed in claim 1, wherein the device comprises optical meanscomprising a camera and at least four emitting diodes disposed on theobject, each of which represents the ends of a parallelogram.
 8. Thedetection method as claimed in claim 1, wherein the object is a pilot'shelmet, the whole of the optical device being installed in an aircraftcockpit.
 9. The method of optical detection as claimed in claim 2,wherein the device comprises optical means comprising a holographicvideo-projector emitting in an image plane sharp luminous patterns atevery point of the zone of sweep corresponding to the space in which theobject may move and at least two identical and mutually parallel linealmatrix sensors, disposed on the object, the four ends of these twosensors forming a parallelogram.
 10. The method of optical detection asclaimed in claim 2, wherein the device comprises optical meanscomprising a camera and at least four emitting diodes disposed on theobject, each of which represents the ends of a parallelogram.
 11. Thedetection method as claimed in claim 2, wherein the object is a pilot'shelmet, the whole of the optical device being installed in an aircraftcockpit.
 12. The detection method as claimed in claim 3, wherein theobject is a pilot's helmet, the whole of the optical device beinginstalled in an aircraft cockpit.
 13. The detection method as claimed inclaim 4, wherein the object is a pilot's helmet, the whole of theoptical device being installed in an aircraft cockpit.
 14. The detectionmethod as claimed in claim 5, wherein the object is a pilot's helmet,the whole of the optical device being installed in an aircraft cockpit.